Hospital beds may be equipped with a support surface having microclimate management (MCM) capability. MCM capability refers to the capability to affect the environment, particularly the temperature and humidity, in the immediate vicinity of the bed occupant. The MCM-capable support surface may be a topper installed on a mattress, or may be the mattress itself. Effective microclimate management can benefit a bed occupant by resisting or mitigating the effects of skin tissue breakdown.
A typical MCM-capable support surface has provisions for receiving and discharging a stream of air. At least that portion of the support surface upon which the occupant rests is vapor permeable. In operation, a stream of air flows through the interior of the support surface. Provided the air is cooler than the occupant's skin, the internal airstream acts as a heat sink to keep the occupant's skin cool, thereby reducing the metabolic demands of the skin tissue and, as a consequence, reducing the likelihood that the occupant will develop pressure ulcers. This mode of heat transfer is proportional to the temperature gradient between the occupant's skin and the airstream (dqDRY/dt=k1ΔT) and is referred to herein as “dry flux”, DF.
In addition, heat transfer from the occupant's skin can cause molecules of perspiration present at the interface between the support surface and the occupant's skin to acquire enough energy to break free, i.e. evaporate, from the liquid perspiration. The liberated molecules migrate through the vapor permeable portion of the occupant support, and are carried away in the internal airstream. The attendant moisture reduction at the skin/surface interface is beneficial because dry skin is less vulnerable to tissue breakdown than wet skin provided the skin is not excessively dry. Moreover, because the evaporation is the result of heat transfer from the occupant, the occupant experiences an evaporative cooling effect above and beyond the above described dry flux. This evaporative mode of heat transfer is proportional to the difference between PH2O,SKIN, the partial pressure of water vapor (perspiration) at the occupant's skin (i.e. at the occupant/surface interface) and PH2O,STREAM, the partial pressure of water vapor in the airstream (dqWET/dt=k2ΔPH2O) and is referred to herein as “wet flux”, WF. The wet flux component of heat transfer materializes only when the occupant is perspiring and depositing liquid phase perspiration at the skin/surface interface.
Typically, the air flowing through the MCM-capable surface is ambient air (e.g. air from a hospital room), unconditioned in the sense that no temperature and/or humidity conditioning has been applied to the air above and beyond the conditioning applied by the hospital heating, ventilating and air conditioning (HVAC) system. As a result, the effectiveness of the MCM-capable support surface is constrained by the properties of the room air. What is needed is a way to selectively achieve enhanced microclimate performance and to govern the degree of enhancement.